1. Field of the Invention
The present invention relates to a non-aqueous pigment ink that is suitable for use with an inkjet recording device, and in particular to a non-aqueous pigment ink that is capable of contributing to power saving, and capable of reducing or eliminating print-through (striking-through), thereby achieving high print density.
2. Description of the Related Art
An inkjet recording system ejects a highly fluid inkjet ink as an ink particle from a very thin head nozzle to record an image on a recording medium, which is positioned to face the nozzle. Because of low noise and ability of high-speed printing, the inkjet recording systems are rapidly becoming widely used in recent years. As inks for use with the inkjet recording systems, so-called non-aqueous pigment inks, which are formed by finely dispersing a pigment in a non-water-soluble solvent, are known.
In recent years, in view of resource environment and energy saving, it is desired to reduce the power consumption of devices, such as printers, as low as possible, and there are ever increasing demands for power-efficient deices for power saving in the field of inkjet printing.
In inkjet recording devices, ink in an ink chamber provided in an inkjet head is ejected from a nozzle when a pressure is applied to the ink chamber. The ink ejected from the nozzle flies with leaving a tail behind it, and there is a time difference and a velocity difference between the leading part and the trailing part of the flying ink. In a low temperature environment, viscosity of the ink increases, and therefore it is necessary to apply a higher driving voltage to the inkjet head for ejecting a desired amount of ink, resulting in increased power consumption. Further, the ink ejection with the higher driving voltage tends to form satellites. The satellites are deposited on a recording medium and degrade the print quality. In order to ensure the print quality, conventionally, in a low temperature environment where the satellites are likely to be formed, a recording operation is started after a so-called warming-up operation to warm the inkjet head is performed.
That is, in a low temperature environment where the satellites are likely to be formed, the recording operation is started after the warming-up operation, and therefore electric power for the warming-up operation is also consumed. Further, the longer the time required for the warming-up operation, the longer the time taken for recording an image. This is a problem not only in view of the electric power but also in view of the operation time of the user. To address this problem from the ink side, it is effective for power saving to reduce the ink viscosity in a low temperature environment. To this end, it is very effective to provide an ink with lower viscosity. The ink viscosity can be reduced by reducing the amount of colorant, or the amount of powder, in the ink. However, this results in lower print density, and thus lower image quality.
For example, an ink using a colorant which is formed by combining a pigment, a dispersant, and a water-soluble resin including two or more primary and/or secondary amino groups in a molecule, which react with reactive functional groups of the dispersant and are chemically-bound to the dispersant, has been proposed in U.S. Pat. No. 7,767,013 (hereinafter, Patent Document 1). This ink has high storage stability, high pigment dispersibility, and high ejection stability with no clogging in the nozzle. A dispersant, in general, stabilizes an ink by repeating adsorption and desorption onto and from the surface of a pigment to achieve an equilibrium state. With the colorant disclosed in Patent Document 1, however, it is necessary to increase the amount of the dispersant to stabilize the ink in a low temperature environment, and it is difficult to reduce the viscosity to a level where the satellites are reduced or eliminated.
Therefore, with the ink of Patent Document 1, it is necessary to warm the ink to reduce the ink viscosity, and there still is the problem of increased power consumption. In particular, in the case where the ink is used with a circulation-type inkjet recording device, the volume of ink to be warmed is larger. This results in higher electric power consumption and longer time taken for warming, and thus longer time taken for outputting the first print. On the other hand, in order to reduce the viscosity, it is necessary to reduce the amount of the dispersant. In this case, however, it is difficult to ensure the pigment dispersion stability.
Use of a hydrocarbon-based non-polar solvent with high boiling point and low viscosity (which will hereinafter be simply referred to as “hydrocarbon-based non-polar solvent”) can provide an ink with low viscosity. When the hydrocarbon-based non-polar solvent is used as an ink solvent, the polarity of the ink solvent is changed, and this may result in poorer pigment dispersion stability. However, it is believed that this problem can be solved by changing the structure of the dispersant. The applicant has proposed, in U.S. Patent Application Publication No. 2011/0046298 (hereinafter, Patent Document 2), a non-aqueous pigment ink containing non-water-soluble resin dispersing particulates, which are capable of dispersing a pigment.